We examine the reasons for discrepancies between two alternative approaches to modeling smallamplitude tides in binary systems. The direct solution (DS) approach solves the governing differential equations and boundary conditions directly, while the modal decomposition (MD) approach relies on a normalmode expansion. Applied to a model for the primary star in the heartbeat system KOI54, the two approaches predict quite different behavior of the secular tidal torque. The MD approach exhibits the pseudosynchronization phenomenon, where the torque due to the equilibrium tide changes sign at a single, welldefined, and theoretically predicted stellar rotation rate. The DS approach instead shows “blurred” pseudosynchronization, where positive and negative torques intermingle over a range of rotation rates. We trace a major source of these differences to an incorrect damping coefficient in the profile functions describing the frequency dependence of the MD expansion coefficients. With this error corrected, some differences between the approaches remain; however, both are in agreement that pseudosynchronization is blurred in the KOI54 system. Our findings generalize to any type of star for which the tidal damping depends explicitly or implicitly on the forcing frequency.
Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher.
Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?
Some links on this page may take you to nonfederal websites. Their policies may differ from this site.

Abstract 
Abstract We describe new functionality in the GYRE stellar oscillation code for modeling tides in binary systems. Using a multipolar expansion in space and a Fourierseries expansion in time, we decompose the tidal potential into a superposition of partial tidal potentials. The equations governing the smallamplitude response of a spherical star to an individual partial potential are the linear, nonradial, nonadiabatic oscillation equations with an extra inhomogeneous forcing term. We introduce a new executable,
gyre _tides , that directly solves these equations within the GYRE numerical framework. Applying this to selected problems, we find general agreement with results in the published literature but also uncover some differences between our direct solution methodology and the modal decomposition approach adopted by many authors. In its present formgyre _tides can model equilibrium and dynamical tides of aligned binaries in which radiative diffusion dominates the tidal dissipation (typically, intermediate and highmass stars on the main sequence). Milestones for future development include incorporation of other dissipation processes, spin–orbit misalignment, and the Coriolis force arising from rotation. 
Abstract We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (Δ
I < 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parametersT _{eff}= 42,000 K, , $\mathrm{log}L/{L}_{\odot}=5.83\pm 0.15$M /M _{⊙}= 50 ± 9, andv sini = 370 ± 40 km s^{−1}. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no Xray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric (e > 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emissionline spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emissionline spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493’s mass and rotation have been enhanced by binary interaction followed by the corecollapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors. 
null (Ed.)ABSTRACT Laplace’s tidal equations govern the angular dependence of oscillations in stars when uniform rotation is treated within the socalled traditional approximation. Using a perturbation expansion approach, I derive improved expressions for the eigenvalue associated with these equations, valid in the asymptotic limit of large spin parameter q. These expressions have a relative accuracy of order q−3 for gravitoinertial modes, and q−1 for Rossby and Kelvin modes; the corresponding absolute accuracy is of order q−1 for all three mode types. I validate my analysis against numerical calculations, and demonstrate how it can be applied to derive formulae for the periods and eigenfunctions of Rossby modes.more » « less

ABSTRACT The time evolution of angular momentum and surface rotation of massive stars are strongly influenced by fossil magnetic fields via magnetic braking. We present a new module containing a simple, comprehensive implementation of such a field at the surface of a massive star within the Modules for Experiments in Stellar Astrophysics (mesa) software instrument. We test two limiting scenarios for magnetic braking: distributing the angular momentum loss throughout the star in the first case, and restricting the angular momentum loss to a surface reservoir in the second case. We perform a systematic investigation of the rotational evolution using a grid of OB star models with surface magnetic fields (M⋆ = 5–60 M⊙, Ω/Ωcrit = 0.2–1.0, Bp = 1–20 kG). We then employ a representative grid of Btype star models (M⋆ = 5, 10, 15 M⊙, Ω/Ωcrit = 0.2, 0.5, 0.8, Bp = 1, 3, 10, 30 kG) to compare to the results of a recent selfconsistent analysis of the sample of known magnetic Btype stars. We infer that magnetic massive stars arrive at the zeroage main sequence (ZAMS) with a range of rotation rates, rather than with one common value. In particular, some stars are required to have closetocritical rotation at the ZAMS. However, magnetic braking yields surface rotation rates converging to a common low value, making it difficult to infer the initial rotation rates of evolved, slowly rotating stars.more » « less

Abstract Asteroseismology of bright stars has become increasingly important as a method to determine the fundamental properties (in particular ages) of stars. The Kepler Space Telescope initiated a revolution by detecting oscillations in more than 500 mainsequence and subgiant stars. However, most Kepler stars are faint and therefore have limited constraints from independent methods such as longbaseline interferometry. Here we present the discovery of solarlike oscillations in α Men A, a nakedeye ( V = 5.1) G7 dwarf in TESS’s southern continuous viewing zone. Using a combination of astrometry, spectroscopy, and asteroseismology, we precisely characterize the solar analog α Men A ( T eff = 5569 ± 62 K, R ⋆ = 0.960 ± 0.016 R ⊙ , M ⋆ = 0.964 ± 0.045 M ⊙ ). To characterize the fully convective M dwarf companion, we derive empirical relations to estimate mass, radius, and temperature given the absolute Gaia magnitude and metallicity, yielding M ⋆ = 0.169 ± 0.006 M ⊙ , R ⋆ = 0.19 ± 0.01 R ⊙ , and T eff = 3054 ± 44 K. Our asteroseismic age of 6.2 ± 1.4 (stat) ± 0.6 (sys) Gyr for the primary places α Men B within a small population of M dwarfs with precisely measured ages. We combined multiple groundbased spectroscopy surveys to reveal an activity cycle of P = 13.1 ± 1.1 yr for α Men A, a period similar to that observed in the Sun. We used different gyrochronology models with the asteroseismic age to estimate a rotation period of ∼30 days for the primary. Alpha Men A is now the closest ( d = 10 pc) solar analog with a precise asteroseismic age from spacebased photometry, making it a prime target for nextgeneration directimaging missions searching for true Earth analogs.more » « less